The structure of hydrated biological objects is being studied in two ways. The surface is being examined by a new wet surface replication method. The wet object is transferred to an aperture limited environmental chamber inside a vacuum evaporator. A thin layer of silicon monoxide is evaporated onto the wet surface. The replica is shadowed and examined in a conventional electron microscope. The method is being applied to whole cells, wet chromatin and metaphase chromosomes. An aperture-limited environmental chamber, placed inside an ETC scanning electron microscope, enables the interior mass distribution of wet chromosomes and other wet objects to be determined. Results on chromosomes are being compared with the results of light-microscope banding and electron microscope critical-point-drying procedures on chromosomes. BIBLIOGRAPHIC REFERENCES: Turner, J.N., Misell, D.L. and Parsons, D.F. (1977). Electron image contrast from carbonaceous material. A comparison of theory with experiment. J. Phys.(D) 10, 1. Basu, S. and Parsons, D.F. (1977). Wet chromosome structure. Biophys. J. 17, 216a.